1. Field of the Invention
The present invention relates generally to an air fuel mixer for the combustor of a gas turbine engine and, in particular, to an air fuel mixer which uniformly mixes fuel and air so as to reduce NOx formed by the ignition of the fuel/air mixture and minimizes auto-ignition and flashback therein.
2. Description of Related Art
The present invention involves an air/fuel mixer for a gas turbine combustor which provides gaseous and/or liquid fuel to the mixing duct so as to be mixed with air to form a uniform air/fuel mixture. Other dual fuel mixers in the art include U.S. Pat. No. 5,351,477 to Joshi et al. and Ser. No. 08/304,341 now U.S. Pat. No. 5,511,375 to Joshi et al., both of which are owned by the assignee of the present invention. Each of these prior art air/fuel mixers, as well as the mixer of the present invention, includes a mixing duct, a set of inner and outer counter-rotating swirlers adjacent to the upstream end of the mixing duct, and a hub separating the inner and outer swirlers to allow independent rotation of the air flow therethrough.
It has been found, however, that these dual fuel mixer designs do not include features to adequately reduce fuel residence time in the mixing duct or otherwise prevent auto-ignition or flashback. Accordingly, a patent application entitled "Dual Fuel Mixer For Gas Turbine Combustor," having Ser. No. 08/581,817, now U.S. Pat. No. 5,680,766 was filed by the assignee of the present invention to address the problems of auto-ignition and flashback. The '817 patent application includes features which energize the boundary layer flow along the mixing duct wall and the centerbody. Nevertheless, it has been found at high pressure and temperature conditions, typical of aircraft engine operation, that liquid fuel can still be entrained into separate regions and remain there long enough to auto-ignite. This can occur through flow separation from the swirler vanes, as well as by flow separation which occurs downstream of the circular fuel jets and air-assist openings disclosed in the '817 application.
Another patent application entitled "Dual Fuel Mixer For Gas Turbine Combustor," having Ser. No. 08/581,818, was further filed by the assignee of the present invention. The mixer design of the '818 application includes features for improving liquid fuel atomization by impinging fuel jets. Once again, at high pressure and temperature conditions, the bulk residence time in the mixing duct has been found to be long enough in some instances to permit liquid fuel to mix with the air flow and auto-ignite. Thus, while improved liquid fuel atomization is desirable, fuel residence time in the mixing duct must be reduced to prevent auto-ignition and/or flashback from occurring at high power operating conditions.
In light of the foregoing, it would be desirable for an air fuel mixer to be developed which better addresses the problems of auto-ignition and flashback while maintaining an emphasis on uniformly mixing liquid and/or gaseous fuel with air so as to reduce NOx formed by the ignition of the air/fuel mixture.